Add V2 session storage service

[kitlangton]

Summary

• add a V2 SessionStorage service boundary with SQL and in-memory implementations
• refactor SessionV2 reads to consume the storage service instead of direct Drizzle queries
• add generic contract tests shared by SQL and memory storage
• document the V2 storage-service migration strategy

Verification

• bun typecheck (packages/opencode)
• bun oxlint packages/opencode/src/v2/session.ts packages/opencode/src/v2/session/storage.ts packages/opencode/src/v2/session/storage-sql.ts packages/opencode/src/v2/session/storage-memory.ts packages/opencode/test/v2/session-storage.test.ts --max-warnings=0
• bun test --timeout 5000 test/v2/session-storage.test.ts
• bun test --timeout 10000 test/session/prompt.test.ts -t "prompt emits v2 prompted and synthetic events"
• git push hook: bun turbo typecheck